Our invention relates generally to printers such as those used in combination with relatively small size computers for the production of so called hard copies and, in particular, to impact printers of the wire dot type having a printing head for printing on a sheet of paper or like material being held against a platen roll. More particularly, our invention pertains to a method of, and means for, adjusting the spacing between the printing head and the platen roll to the thickness of the sheet to be printed upon in printers of the kind defined.
The printers of the class under consideration are usually put to use with sheets of paper of various thicknesses. The spacing between printing head and platen roll must therefore be readily adjustable to the particular thickness of the sheets to be printed upon for the production of high quality printings. This requirement necessarily implies that the printer must be so constructed that the printing head is adjustably movable toward and away from the platen roll, besides being movable back and forth along the axis of the platen roll.
Conventionally, for such adjustable movement of the printing head toward and away from the platen roll, a bidirectional electric motor such as a stepper motor has been coupled via a friction clutch to an eccentric shaft rotatably extending through a head carriage. The rotation of the eccentric shaft has been translated into the travel of the head carriage, with the printing head mounted fast thereon, toward and away from the platen roll. A sheet presser is also mounted fast to the head carriage for pressing the sheet against the platen roll.
For the adjustment of the head-platen spacing the head carriage has been first retracted away from the platen roll into abutment against a limit stop. Then the head carriage has been driven forward until the sheet presser comes to butt on the platen roll via the sheet to be printed upon.
We object to such conventional means for, and conventional method of, adjusting the head-platen spacing for more reasons than one. First, the limit stop that determines the fully retracted position of the printing head has of necessity been formed of a part totally independent of the platen roll, with several other parts interposed therebetween. The limit stop has had to be positioned highly exactly with respect to the platen roll as the conventional method relies on the fully retracted position of the printing head as the reference position for the determination of the head-platen spacing. Such exact placement of the independent limit stop has made the printers of this kind very costly in construction and troublesome in assemblage.
Another objection to the prior art arises when the sheet presser is forced against the platen roll, either directly or via the sheet to be printed upon, by the electric motor coupled to the eccentric shaft via the friction clutch. Then the friction clutch develops a slip until the motor is set out of rotation. The sheet presser has thus been urged against the platen roll under the full driving force of the motor combined with the weights of the printing head, head carriage, eccentric shaft and so forth. Consequently, not only the sheet presser but also the head carriage, eccentric shaft and other parts have been prone to suffer elastic deformation or, in the worst case, permanent displacement. Such deformation or displacement has made it difficult or impossible to accurately adjust the head-platen spacing to the thickness of the sheet.